The present invention relates to lubricating oil compositions and more specifically to those with excellent thermal/oxidation stability and an excellent effect of inhibiting the increases of the viscosity and acid number even in the presence of NOx, thus suitable for internal combustion engines. The present invention also relates to lubricating oil compositions with excellent high-temperature detergency and base number retention properties and those having such excellent properties and still highly capable of achieving the effect of inhibiting wear caused by soot contamination in the composition and inhibit an exhaust-gas after-treatment device from being adversely affected, thus suitable for diesel and direct injection engines equipped with such a device.
The lubricating oil used in internal combustion engines such as automobile engines has been required to have thermal/oxidation stability for use under severe conditions for a long period of time. Conventional lubricating oils for internal combustion engines comprises a high performance base oil such as a highly refined mineral oil or synthetic oil, blended with zinc dithiophosphate (ZDTP), molybdenum dithiocarbamate (MoDTC), or a phenolic anti-oxidant (see, for example, Japanese Patent Laid-Open Publication Nos. 4-36391, 63-223094, 8-302378, and 9-3463).
However, for recent more severe service conditions against the lubricating oil for internal combustion engines and reduction in waste oil disposal, the lubricating oil is further required to have a longer service life. However, conventional high performance base oils are not necessarily sufficient in their own thermal/oxidation stability. Alternatively, even though an anti-oxidant is added in a larger amount, there is a certain limit to improve thermal/oxidation stability. Thus, a further improvement must be sought in order to meet such requirements. In particular, since in the presence of NOx, the viscosity and acid number of a lubricating oil are significantly increased due to oxidation degradation, a long-life lubricating oil has been demanded which can sufficiently suppress the oxidation degradation.
In recent years, a lubricating oil composition has been proposed which contains a phosphorus compound with less or no sulfur in place of ZDTP (see, for example, Japanese Patent Laid-Open Publication No. 2002-294271). However, it must be further improved in suppression of viscosity or acid number increase in the presence of NOx, and a lubricating oil containing ZDTP must be significantly improved in this regard.
Further, in the background of environment issues, the regulation against exhaust gas emission throughout the world has become tightened year by year. In particular, the exhaust gas from a diesel engine is urgently required to be decreased in NOx or suspended particle matters (SPM). Conventionally, in order to decrease the emission of these materials, it has been studied to introduce means for decreasing exhaust gas, such as a high-pressure injection system, an exhaust-gas recycling (EGR) system, an oxidation catalyst, a diesel particulate filter (DPF), or a NOx adsorber, into an diesel engine. It is known that among these emission control means, an exhaust-gas after-treatment device such as an oxidation catalyst, NOx adsorber, and DPF in particular are shortened in service life depending on the composition of the lubricating oil to be used. For example, in the case of using a lubricating oil containing zinc dialkyldithiophosphate (hereinafter referred to as “ZnDTP”) that is effective as an anti-wear agent or anti-oxidant (peroxide decomposer), it is known that the zinc contained in ZnDTP forms an oxide or a phosphoric acid salt during the combustion process and there is the possibility that such an oxide or salt may degrade the purification performance of the exhaust-gas after-treatment device because it accumulates on the catalyst surface or in the filter. Therefore, a lubricating oil for an engine equipped with the exhaust-gas after-treatment device as described above desirously contains no ZnDTP or a small amount thereof even if added. Further, a metallic detergent and sulfur components are preferably decreased as much as possible because a sulfuric acid salt and an oxide accumulate in the form of ash and thus are likely to cause the foregoing problems.
However, in a diesel engine, in particular that equipped with an EGR, a large amount of soot contaminates the lubricating oil. Therefore, when the content of the ZnDTP, metallic detergent or sulfur components in the lubricating oil is simply decreased, there is a concern that the valve train wear would be increased and high-temperature detergency such as piston detergency would be poor. With regard to a direct injection gasoline engine, there also arises a concern that soot contamination in the lubricating oil would cause adverse affects similar to the foregoing and the formation of deposit on the combustion chamber and valves. Therefore, when the contents of the ZnDTP, metallic detergent and sulfur components are decreased, it is necessary to research and develop new measures for compensating the detergency and anti-wear properties degraded in connection with the decreased contents.
As a lubricating oil composition for an engine equipped with an exhaust-gas after-treatment device, Japanese Patent Laid-Open Publication No. 2000-256690 proposes a diesel engine oil composition, the sulfated ash content of which is held down to 0.7 percent by mass or less. Japanese Patent Laid-Open Publication No. 2001-279287 proposes an engine oil containing a dispersant type viscosity index improver for significantly improving the detergency when soot contaminates the oil. As an oil which is improved in anti-wear properties when soot contamination occurs therein, Japanese Patent Laid-Open Publication No. 2004-10799 proposes a lubricating oil composition containing a dispersant type viscosity index improver with a specific molecular weight similar to that disclosed in No. 2001-279287. However, these proposals are not necessarily sufficiently improved in high-temperature detergency or base number retention properties as a metallic detergent is decreased and do not discuss about high-temperature detergency as ZnDTP is decreased or about anti-wear properties as soot contaminates the oils. As the result, further consideration is needed in order to maintain or improve high-temperature detergency and base number retention properties at or to a higher level and inhibit wear caused by soot contamination in the oil, occurring significantly when the content of ZnDTP is decreased.